Some microwave applications require a strong magnetic field that varies in strength along the path of a radiation-generating electron beam. For example, a gyrotron-type device requires an abrupt step-like magnetic field that varies along an electron beam path. Such gyrotron-type devices comprise a tandem of coaxial chambers made of permanent magnet structures wherein each chamber generates a uniform axial magnetic field having a predetermined magnitude and direction. Since each chamber in tandem can be configured to generate a magnetic field different from the other chamber, the magnetic field at the juncture of the two chambers may be configured to have a step profile.
Many gyrotron-type devices do not require that there be a perfect step profile as a function of distance along the axis of the bi-chambered device. It has been recognized by those skilled in the art, however, that the ability to produce a gyrotron-type device having such a perfect step profile would be a useful addition to the repertoire of field forms attainable with permanent magnets.
A first step in the attainment of such a device was disclosed in an article entitled "Magnetic-field source for bi-chambered electron-beam devices," authored by the applicants in the Journal of Applied Physics No. 67 (9), on May 1, 1990. In this article it was disclosed that the broad transition in the magnetic field values between the juncture of the two chambers can be sharpened by implementing a radially magnetized permanent magnet ring at the juncture of the chambers. The ring was placed in the interior rather than on the outside of the device to prevent unwanted polarization of the iron pole pieces at the joints as the magnetic ring produces a smaller field at its exterior.
Smoothing of the field transition can also be accomplished by the addition of axially magnetized rings whose fields are equivalent to the changes resulting from an alteration in the remanence in the supply magnet at the location in question.
Such field transition sharpening techniques are effective on these devices only when the required fields are on the order of, but no greater than, one-half the coercivity of the permanent magnet materials that comprise the structure.
Consequently, those skilled in the art recognize the need for a device that can provide a stepped internal field having a magnitude greater than that of one-half the coercivity of the permanent magnet materials that comprise the shell without adding to the overall size and cost of the device.